Traditional phenotypic criteria for classification of streptococci include both hemolytic reactions and Lancefield serological groupings. However, with taxonomic advances, it is now known that unrelated species of β-hemolytic (defined as the complete lysis of sheep erythrocytes in agar plates) streptococci (BHS) may produce identical Lancefield antigens and that strains genetically related at the species level may have heterogeneous Lancefield antigens. In spite of these exceptions to the traditional rules of streptococcal taxonomy, hemolytic reactions and Lancefield serological tests can still be used to divide streptococci into broad categories as a first step in identification of clinical isolates. Ruoff, K. L., R. A. Whiley, and D. Beighton. 1999. Streptococcus. In P. R. Murray, E. J. Baron, M. A. Pfaller, F. C. Tenover, and R. H. Yolken (eds.), Manual of Clinical Microbiology. American Society of Microbiology Press, Washington D.C.
β-hemolytic isolates with Lancefield group A, C, or G antigen can be subdivided into two groups: large-colony (>0.5 mm in diameter) and small-colony (<0.5 mm in diameter) formers. Large-colony-forming group A (Streptococcus pyogenes), C, and G strains are “pyogenic” streptococci replete with a variety of effective virulence mechanisms. Streptococcus agalactiae (group B) is still identified reliably by its production of Lancefield group B antigen or other phenotypic traits.
Similarities between BHS species include not only virulence factors, but also disease manifestations. Included in the latter are pneumonia, arthritis, abscesses, rhinopharyngitis, metritis, puerperal sepsis, neonatal septicemia, wound infections, meningitis, peritonitis, cellulitis, pyoderma, necrotizing fasciitis, toxic shock syndrome, septicemia, infective endocarditis, pericarditis, glomerulonephritis, and osteomyelitis.
Streptococcus pyogenes are Gram-positive diplococci that colonize the pharynx and skin of humans, sites that then serve as the primary reservoir for this organism. An obligate parasite, this bacterium is transmitted by either direct contact of respiratory secretions or by hand-to-mouth. The majority of Streptococcus pyogenes infections are relatively mild illnesses, such as pharyngitis or impetigo. Currently, there are anywhere from twenty million to thirty-five million cases of pharyngitis alone in the U.S., costing about $2 billion for physician visits and other related expenses. Additionally, nonsuppurative sequelae such as rheumatic fever, scarlet fever, and glomerulonephritis result from Streptococcus pyogenes infections. Globally, acute rheumatic fever (ARF) is the most common cause of pediatric heart disease (1997. Case definitions for Infectious Conditions Under Public Health Surveillance. CDC.).
From the initial portals of entry, pharynx, and skin, Streptococcus pyogenes can disseminate to other parts of the body where bacteria are not usually found, such as the blood, deep muscle and fat tissue, or the lungs, and can cause invasive infections. Two of the most severe but least common forms of invasive Streptococcus pyogenes disease are necrotizing fasciitis and streptococcal toxic shock syndrome (STSS). Necrotizing fasciitis (described in the media as “flesh-eating bacteria”) is a destructive infection of muscle and fat tissue. STSS is a rapidly progressing infection causing shock and injury to internal organs such as the kidneys, liver, and lungs. Much of this damage is due to a toxemia rather than localized damage due to bacterial growth.
In 1995, invasive Streptococcus pyogenes infections and STSS became mandated reportable diseases. In contrast to the millions of individuals that acquire pharyngitis and impetigo, the U.S. Centers for Disease Control and Prevention (CDC) mandated case reporting indicates that in 1997 there were from 15,000 to 20,000 cases of invasive Streptococcus pyogenes disease in the United States, resulting in over 2,000 deaths (1997. Case definitions for Infectious Conditions Under Public Health Surveillance. CDC.). Other reports estimate invasive disease to be as high as 10-20 cases per 100,000 individuals per year (Stevens, D. L. 1995. Streptococcal toxic-shock syndrome: spectrum of disease, pathogenesis, and new concepts in treatment. Emerg Infect Dis. 1:69-78). More specifically, of the 15,000 to 20,000 cases of invasive disease, 1,100 to 1,500 are cases of necrotizing fasciitis and 1,000 to 1,400 are cases of STSS, with a 20% and 60% mortality rate, respectively. Also included in serious invasive disease are cases of myositis, which carries a fatality rate of 80% to 100%. An additional 10% to 15% of individuals die with other forms of invasive group A streptococcal disease. These numbers have increased since case reporting was initiated in 1995 and reflect a general trend that has occurred over the past decade or two. Additionally, it is commonly agreed that the stringency of the case definitions results in lower and, thus, misleading numbers, in that many cases are successfully resolved due to early diagnosis and treatment before the definition has been met.
While Streptococcus pyogenes remains sensitive to penicillin and its derivatives, treatment does not necessarily eradicate the organism. Approximately 5% to 20% of the human population are carriers depending on the season (Stevens, D. L. 1995. Streptococcal toxic-shock syndrome: spectrum of disease, pathogenesis, and new concepts in treatment. Emerg Infect Dis. 1:69-78), despite antibiotic therapy. The reasons for this are not totally clear and may involve a variety of mechanisms. In cases of serious invasive infections, treatment often requires aggressive surgical intervention. For those cases involving STSS or related disease, clindamycin (a protein synthesis inhibitor) is the preferred antibiotic as it penetrates tissues well and prevents exotoxin production. There are reports of some resistance to tetracycline, sulfa, and most recently, erythromycin. Clearly, there remains a need for compositions to prevent and treat β-hemolytic infection.
Numerous virulence factors have been identified for Streptococcus pyogenes, some secreted and some surface localized. Although it is encapsulated, the capsule is composed of hyaluronic acid and is not suitable as a candidate antigen for inclusion in immunogenic compositions, since it is commonly expressed by mammalian cells and is nonimmunogenic (Dale, J. B., R. G. Washburn, M. B. Marques, and M. R. Wessels. 1996. Hyaluronate capsule and surface M protein in resistance to opsonization of group A streptococci. Infect Immun. 64:1495-501). The T antigen and Group Carbohydrate are other candidates, but may also elicit cross-reactive antibodies to heart tissue. Lipoteichoic acid is present on the surface of Streptococcus pyogenes, but raises safety concerns similar to LPS.
The most abundant surface proteins fall into a family of proteins referred to as M or “M-like” proteins because of their structural similarity. While members of this class have similar biological roles in inhibiting phagocytosis, they each have unique substrate binding properties. The best characterized protein of this family is the helical M protein. Antibodies directed to homologous M strains have been shown to be opsonic and protective (Dale, J. B., R. W. Baird, H. S. Courtney, D. L. Hasty, and M. S. Bronze. 1994. Passive protection of mice against group A streptococcal pharyngeal infection by lipoteichoic acid. J Infect Dis. 169:319-23, Dale, J. B., M. Simmons, E. C. Chiang, and E. Y. Chiang. 1996. Recombinant, Ellen, R. P., and R. J. Gibbons. 1972. M protein-associated adherence of Streptococcus pyogenes to epithelial surfaces: prerequisite for virulence. Infect Immun. 5:826-830.). Complicating the use of M protein as a candidate antigen is the fact that there have been approximately 100 different serotypes of M protein identified with several more untyped. Typically, the Class I M serotypes, exemplified by serotypes M1, M3, M6, M12, and M18, are associated with pharyngitis, scarlet fever, and rheumatic fever and do not express immunoglobulin binding proteins. Class II M serotypes, such as M2 and M49, are associated with the more common localized skin infections and the sequelae glomerulonephritis, and do express immunoglobulin binding proteins (Podbielski, A., A. Flosdorff, and J. Weber-Heynemann. 1995. The group A streptococcal virR49 gene controls expression of four structural vir regulon genes. Infect Immun. 63:9-20). It is important to note that there is little, if any, heterologous cross-reactivity of antibodies to M serotypes. Equally important is the role these antibodies play in rheumatic fever. Specific regions of M protein elicit antibodies that cross react with host heart tissue, causing or at least correlating with cellular damage (Cunningham, M. W., and A. Quinn. 1997. Immunological crossreactivity between the class I epitope of streptococcal M protein and myosin. Adv Exp Med Biol. 418:887-921, Quinn, A., K. Ward, V. A. Fischetti, M. Hemric, and M. W. Cunningham. 1998. Immunological relationship between the class I epitope of streptococcal M protein and myosin. Infect Immun. 66:4418-24.).
M and M-like proteins belong to a large family of surface localized proteins that are defined by the sortase-targeted LPXTG motif (Mazmanian, S. K., G. Liu, H. Ton-That, and O. Schneewind. 1999. Staphylococcus aureus sortase, an enzyme that anchors surface proteins to the cell wall. Science. 285:760-3, Ton-That, H., G. Liu, S. K. Mazmanian, K. F. Faull, and O. Schneewind. 1999. Purification and characterization of sortase, the transpeptidase that cleaves surface proteins of Staphylococcus aureus at the LPXTG motif. Proc Natl Acad Sci USA. 96:12424-12429). This motif, located near the carboxy-terminus of the protein, is first cleaved by sortase between the threonine and glycine residues of the LPXTG motif. Once cleaved, the protein is covalently attached via the carboxyl of threonine to a free amide group of the amino acid cross-bridge in the peptidoglycan, thus permanently attaching the protein to the surface of the bacterial cell. Included in this family of sortase-targeted proteins are the C5a peptidase (Chen, C. C., and P. P. Cleary. 1989. Cloning and expression of the streptococcal C5a peptidase gene in Escherichia coli: linkage to the type 12 M protein gene. Infect. Immun. 57:1740-1745, Chmouryguina, I., A. Suvorov, P. Ferrieri, and P. P. Cleary. 1996. Conservation of the C5a peptidase genes in group A and B streptococci. Infect. Immun. 64:2387-2390), adhesins for fibronectin (Courtney, H. S., Y. Li, J. B. Dale, and D. L. Hasty. 1994. Cloning, sequencing, and expression of a fibronectin/fibrinogen-binding protein from group A streptococci. Infect Immun. 62:3937-46, Fogg, G. C., and M. G. Caparon. 1997. Constitutive expression of fibronectin binding in Streptococcus pyogenes as a result of anaerobic activation of rofA. J Bacteriol. 179:6172-80, Hanski, E., and M. Caparon. 1992. Protein F, a fibronectin-binding protein, is an adhesion of the group A streptococcus Streptococcus pyogenes. Proc Natl Acad Sci., USA. 89:6172-76, Hanski, E., P. A. Horwitz, and M. G. Caparon. 1992. Expression of protein F, the fibronectin-binding protein of Streptococcus pyogenes JRS4, in heterologous streptococcal and enterococcal strains promotes their adherence to respiratory epithelial cells. Infect Immun. 60:5119-5125), vitronectin, and type IV collagen, and other M-like proteins that bind plasminogen, IgA, IgG, and albumin (Kihlberg, B. M., M. Collin, A. Olsen, and L. Bjorck. 1999. Protein H, an antiphagocytic surface protein in Streptococcus pyogenes. Infect Immun. 67:1708-14).
Numerous secreted proteins have been described, several of which are considered to be toxins. Most Streptococcus pyogenes isolates from cases of serious invasive disease and streptococcal toxic shock syndrome (STSS) produce streptococcal pyogenic exotoxins (SPE) A and C (Cockerill, F. R., 3rd, R. L. Thompson, J. M. Musser, P. M. Schlievert, J. Talbot, K. E. Holley, W. S. Harmsen, D. M. Ilstrup, P. C. Kohner, M. H. Kim, B. Frankfort, J. M. Manahan, J. M. Steckelberg, F. Roberson, and W. R. Wilson. 1998. Molecular, serological, and clinical features of 16 consecutive cases of invasive streptococcal disease. Southeastern Minnesota Streptococcal Working Group. Clin Infect Dis. 26:1448-58). Other pyogenic exotoxins have also been identified in the genomic Streptococcus pyogenes sequence completed at the University of Oklahoma, submitted to GenBank and assigned accession number AE004092, and have been characterized (Proft, T., S. Louise Moffatt, C. J. Berkahn, and J. D. Fraser. 1999. Identification and Characterization of Novel Superantigens from Streptococcus pyogenes. J Exp Med. 189:89-102). Other toxins such as Toxic Shock Like Syndrome toxin, Streptococcal Superantigen (Reda, K. B., V. Kapur, D. Goela, J. G. Lamphear, J. M. Musser, and R. R. Rich. 1996. Phylogenetic distribution of streptococcal superantigen SSA allelic variants provides evidence for horizontal transfer of ssa within Streptococcus pyogenes. Infect Immun. 64:1161-5), and Mitogenic Factor (Yutsudo, T., K. Okumura, M. Iwasaki, A. Hara, S. Kamitani, W. Minamide, H. Igarashi, and Y. Hinuma. 1994. The gene encoding a new mitogenic factor in a Streptococcus pyogenes strain is distributed only in group A streptococci. Infection and Immunity. 62:4000-4004) play lesser-defined roles in disease. Streptolysin O could also be considered a possible candidate antigen, because it causes the release of IL-β release. In addition, a variety of secreted enzymes have also been identified that include the Cysteine protease (Lukomski, S., C. A. Montgomery, J. Rurangirwa, R. S. Geske, J. P. Banish, G. J. Adams, and J. M. Musser. 1999. Extracellular cysteine protease produced by Streptococcus pyogenes participates in the pathogenesis of invasive skin infection and dissemination in mice. Infect Immun. 67:1779-88, Matsuka, Y. V., S. Pillai, S. Gubba, J. M. Musser, and S. B. Olmsted. 1999. Fibrinogen cleavage by the Streptococcus pyogenes extracellular cysteine protease and generation of antibodies that inhibit enzyme proteolytic activity. Infect Immun. 67:4326-33), Streptokinase (Huang, T. T., H. Malke, and J. J. Ferretti. 1989. The streptokinase gene of group A streptococci: cloning, expression in Escherichia coli, and sequence analysis. Mol Microbiol. 3:197-205, Nordstrand, A., W. M. McShan, J. J. Ferretti, S. E. Holm, and M. Norgren. 2000. Allele substitution of the streptokinase gene reduces the nephritogenic capacity of group A streptococcal strain NZ131. Infect Immun. 68:1019-25), and Hyaluronidase (Hynes, W. L., A. R. Dixon, S. L. Walton, and L. J. Aridgides. 2000. The extracellular hyaluronidase gene (hylA) of Streptococcus pyogenes. FEMS Microbiol Lett. 184:109-12, Hynes, W. L., L. Hancock, and J. J. Ferretti. 1995. Analysis of a second bacteriophage hyaluronidase gene from Streptococcus pyogenes: evidence for a third hyaluronidase involved in extracellular enzymatic activity. Infect Immun. 63:3015-20).
Given the number of known virulence factors produced by Streptococcus pyogenes, it is clear that an important characteristic for a successful β-hemolytic streptococcal immunogenic composition would be its ability to stimulate a response that would prevent or limit colonization early in the infection process. This protective response would either block adherence and/or enhance the clearance of cells through opsonophagocytosis. Antibodies to M protein have been shown to be opsonic and provide a mechanism to overcome the anti-phagocytic properties of the protein (Jones, K. F., and V. A. Fischetti. 1988. The importance of the location of antibody binding on the M6 protein for opsonization and phagocytosis of group A M6 streptococci. J Exp Med. 167:1114-23) in much the same way that anti-serotype B capsular antibodies have demonstrated protection from disease caused by Haemophilus influenzae B (Madore, D. V. 1998. Characterization of immune response as an indicator of Haemophilus influenzae type b vaccine efficacy. Pediatr Infect Dis J. 17:S207-10). In addition, antibodies specific to Protein F have been shown to block adherence and internalization by tissue culture cells (Molinari, G., S. R. Talay, P. Valentin-Weigand, M. Rohde, and G. S. Chhatwal. 1997. The fibronectin-binding protein of Streptococcus pyogenes, SfbI, is involved in the internalization of group A streptococci by epithelial cells. Infect Immun. 65:1357-63).
There remains a need to develop immunogenic compositions and methods to prevent or ameliorate infections caused by β-hemolytic streptococci, including groups A, B, C and G. There also remains a need to provide immunogenic compositions which provide immunity to a broad range of BHS bacteria.